The compounds disclosed herein are synthetic arylmethanesulphinyl derivatives related to the biological and chemical analogs of modafinil. Modafinil, C15H15NO2S, also known as 2-(benzhydrylsulphinyl)acetamide, or 2-[(diphenylmethyl)sulphinyl]acetamide, a synthetic acetamide derivative with wake-promoting activity, has been described in French Patent No. 78 05 510 and in U.S. Pat. No. 4,177,290 (“the '290 patent”). All these molecules share in common, in their structure, a stereogenic center at the sulfur atom and therefore exist as pair of enantiomers. Both enantiomers may exhibit differential stereochemically dependent metabolism and enzyme inhibition. Due to FDA and Registration Agencies policy statement regarding the development of new stereoisomeric drugs, both enantiomers of pharmaceutically interesting chiral sulphoxides need to be synthesized and their biological activity determined. The synthesis of chiral sulphoxides with high enantiomeric purity is presently of interest.
The enantiomers may be processed by chiral resolution methods, which imply salt formation of an acid racemate compounds. The resulting diastereoisomers have to be separated and converted into the optically pure enantiomers by hydrolysis or bond cleavage. These methods are generally time consuming. As an example, such a method was applied to modafinil enantiomers (U.S. Pat. No. 4,927,855). The levorotary isomer of modafinic acid was obtained with very poor yields of about 21% from racemic modafinic acid and had to be further processed by esterification and amidation steps, before the single enantiomer of the required amide modafinil was obtained.
Considering alternative ways of obtaining enantiomerically pure arylmethanesulphinyl derivatives various metal-catalyzed enantioselective oxidations or stoichiometric transition-metal-promoted asymmetric reactions were described in the literature to prepare chiral sulphoxides by chemical oxidation of the corresponding sulphides (Kagan H. B. In “Catalytic Asymmetric Synthesis”; Ojima I., Ed. VCH: New York 1993, 203-226; Madesclaire M., Tetrahedron 1986; 42, 5459-5495; Procter D., J. Chem. Soc. PerkinTrans 1999; 641-667; Fernandez I. et al., Chem. Review 2003; 103(9): 3651-3706. Metal-catalyzed enantioselective oxidations involve a metal catalyst complexed with a chiral ligand such as diethyl tartrate, C2-symmetric diols or C3-symmetric chiral trialkanolamine titanium(IV) complexes, C3-symmetric trialkanolamine zirconium(IV) complex, chiral (salen) manganese(III) complex, chiral (salen) vanadium(IV) complex in the presence of various oxidants such as H2O2 tert-butyl hydroperoxide, cumene hydroperoxide. Methods based on chiral oxaziridines have also been used in the chemical oxidation of sulphides.
Some enzymatic methods for the asymmetric synthesis of fine chemicals were described in Faber K. in “Biotransformations in Organic Chemistry”, Springer Ed. 3rd ed. 1997 and reviewed by Fernandez I. et al. (Chem. Rev. 2003; 103(9): 3651-3706). As an example, thioethers can be asymmetrically oxidized both by bacteria [e.g. Corynebacterium equi (Ohta H. et al. Agrig. Biol. Chem. 1985; 49: 671), Rhodococcus equi (Ohta H. et al. Chem. Lett. 1989; 625)] and fungi [Helminthosporium sp., Mortieralla isabellina sp. (Holland H L. et al. Bioorg. Chem. 1983; 12:1)]. A large variety of aryl alkyl thioethers were oxidized to yield sulphoxides with good to excellent optical purity [(Ohta H. et al. Agrig. Biol. Chem. 1985; 49:671; Abushanab E. et al., Tetrahedron Lett. 1978; 19:3415; Holland H L. et al. Can. J. Chem. 1985; 63:1118)]. Mono-oxygenases and peroxidases are important class of enzymes able to catalyse the oxidation of a variety of sulphides into sulphoxides (Secundo S. et al. Tetrahedron: Asymmetry 1993; 4:1981). The stereochemical outcome of the enzymatic reactions has been shown to be highly dependant on the sulphide structure.
As an other alternative of the enzymatic approach, optically pure methyl arylsulphinylacetates with high enantiomeric excess (>98%) obtained by lipase-catalyzed resolution of the corresponding racemate were also described (Burgess K. et al. Tetrahedron Letter 1989; 30: 3633).
As an enantioselective oxidation method, an asymmetric sulphide oxidation process has been developed by Kagan and co-workers (Pitchen, P; Deshmukh, M., Dunach, E.; Kagan, H. B.; J. Am. Chem. Soc., 1984; 106, 8188-8193). In this process for asymmetric oxidation of sulphides to sulphoxides, the oxidation is performed by using tert-butyl hydroperoxide (TBHP) as oxidizing agent in the presence of one equivalent of a chiral complex obtained from Ti(OiPr)4/(+) or (−) diethyl tartrate/water in the molar ratio 1:2:1.
The general procedure for sulphide oxidation according to Kagan comprises first preforming the chiral complex at room temperature in methylene chloride before adding the sulphide. Then, the oxidation reaction is effected at −20° C. in the presence of tert-butyl hydroperoxide.
The direct oxidation of a variety of sulphides, notably for arylalkyl sulphides into optically active sulphoxides, with an enantiomeric excess (ee), in the range of 80-90%, can be achieved by this method.
More specifically, Kagan and co-workers reported that sulphoxide products could be obtained with high enantioselectivity when sulphides bearing two substituents of very different size were subjected to an asymmetric oxidation. For instance, when aryl methyl sulphides were subjected to oxidation, it was possible to obtain the aryl methyl sulphoxides in an enantiomeric excess (ee) of more than 90%.
Notably, cyclopropylphenyl sulphoxide is formed with 95% ee by this method.
However, asymmetric oxidation of functionalized sulphides, notably those bearing an ester function, was found to proceed with moderate enantioselectivity under these conditions.
Thus, compounds bearing on the stereogenic center, i.e. the sulphur atom, an alkyl moiety with an ester function close to the sulphur atom, such as methylphenylthioacetate, ethylmethylthioacetate and methylmethylthiopropanoate, are reported with ee of only 63-64% (H. B. Kagan, Phosphorus and Sulphur, 1986; 27, 127-132).
Similarly, oxidation of the aryl methyl sulphides with a methyl ester function in the ortho position of the aryl group yields low enantiomeric excess (60%) and yield (50%) as compared to the para substituted compound (ee 91%, yield 50%) or to the p-tolyl methyl sulphide (ee 91%, yield 90%) (Pitchen, P et al., J. Am. Chem. Soc., 1984; 106, 8188-8193).
Hence, even when the substituents on the sulphur atom differ in size, the presence of an ester function close to the sulphur atom strongly affects the enantioselectivity of the asymmetric oxidation.
These results also show that the enantioselectivity of this process is highly depends on the structure and notably on the functionality of the substrate. More specifically, oxidation of sulphides bearing an ester function close to the sulphur gives little asymmetric induction.
Similarly, none of the enantioselective reactions so far reported in the literature deals with substrates bearing an acetamide or acetic acid moiety directly linked to the sulphur atom.
There have been attempts to improve the enantioselectivity by modifying some conditions for asymmetric oxidation of sulphides. For example, Kagan and co-workers (Zhao, S.; Samuel O.; Kagan, H. B., Tetrahedron 1987; 43, (21), 5135-5144) found that the enantioselectivity of oxidation could be enhanced by using cumene hydroperoxide instead of tert-butyl hydroperoxide (ee up to 96%). However, these conditions do not solve the problem of oxidation of sulphides bearing ester, amide or carboxylic acid functions close to the sulphur atom.
Thus, the applicant obtained crude (−)-modafinil with a typical enantiomeric excess of at most about 42% with the above method using the conditions described by Kagan H. B. (Organic Syntheses, John Wiley and Sons INC. ed. 1993, vol. VIII, 464-467).
H. Cotton and co-workers (Tetrahedron: Asymmetry 2000; 11, 3819-3825) recently reported a synthesis of the (S)-enantiomer of omeprazole via asymmetric oxidation of the corresponding prochiral sulphide. Omeprazole, also called 5-methoxy-2-[[(4-methoxy-3,5-dimethyl-2pyridinyl)methyl]-sulphinyl]-1H-benzimidazole is represented by the following formula:

The asymmetric oxidation was achieved by titanium-mediated oxidation with cumene hydroperoxide (CHP) in the presence of (S,S)-(−) diethyl tartrate [(S,S)-(−)-DET]. The titanium complex was prepared in the presence of the prochiral sulphide and/or during a prolonged time and by performing the oxidation in the presence of N,N-diisopropylethylamine. An enantioselectivity of >94% was obtained by this method, whereas the Kagan's original method gives a modest enantiomeric excess of the crude product (30%).
According to the authors, the improved enantioselectivity of this process applied to omeprazole only is probably linked to the presence of benzimidazole or imidazole group adjacent to sulphur, which steers the stereochemistry of formed sulphoxide. The authors also suggested using this kind of functionality as directing groups when synthesizing chiral sulphoxides in asymmetric synthesis.
Hence, this publication is essentially focused on omeprazole, a pro-chiral sulphide bearing substituents of approximately the same size, and including an imidazole group which is described to play an important role in the asymmetric induction.
Therefore, there is a need for an improved enantioselective process for the manufacture of optically pure arylmethanesulphinyl derivatives which overcomes the drawbacks of the prior art and, in particular, allows high yields.